Reinforced plastic parts or products are composite structures in which a resin, either thermosetting or thermoplastic, is combined with a reinforcing member to strengthen and improve various properties of the plastic matrix. Particularly useful processes include resin transfer molding (RTM) and structural reaction injection molding (S-RIM). The former usually employ epoxies or polyester resins, although other thermosetting resins can also be employed. Polyurethanes are employed in S-RIM systems.
Both processes commonly employ a fiber preform or mold charge blank which may approximate the shape of the composite products or at least provides a structural backbone for the product. The mold charge blank is placed inside of a multiple-piece mold, after which the liquid resin or reaction components are transferred into the closed mold to impregnate the reinforcement material and fill all void space within the mold. After the necessary reaction and cure time for the particular resin, the mold is opened, and the finished composite product is removed.
Such composite products are currently of particular interest and importance in a variety of industrial applications, such as the automotive industry, where features such as weight reduction, strength, appearance, durability, flexibility, and consolidation of parts are significant design considerations. Automobile bumpers, for instance, made of polyurethanes reinforced with fibers from a mold charge blank, provide improved performance characteristics, particularly strength, and may also reduce the total weight and number of parts required.
Fiber mold charge blanks are manufactured separately and are supplied to the mold for combination with the liquid resin component(s). A variety of methods have been employed for making mold charge blanks, including the spraying of chopped fibers onto a molded surface and the forming of reinforcing mat or fabric in desired thickness and layers into the shape of the mold charge blank. The fiber material is normally combined with a thermoplastic binder material, and the "laid up" material is then placed in a preheat oven, transferred to a mold, pressed by the mold to the desired shape, and cooled to produce the actual mold charge blank which can be handled and stored until final manufacture. The binder material stiffens the mold charge blank, giving it appropriate structural integrity for liquid molding in the forming tool or handlability for storage or transfer to the final mold where resin transfer occurs.
Advances in automating the production of mold charge blanks have made the final molding process a primary limiting factor in the productivity of relatively large products requiring molds having dimensions on the order of four feet by eight feet. The final molding process is severely limited in a time sense due primarily to the reaction and cure time of the resins, which may be on the order of two or three minutes for S-RIM and up to fifteen minutes for epoxy RTM. Further, due to the size, weight, and complexity of the molds, most presses for such relatively large products employ very large and relatively expensive hydraulic systems.
In the case of relatively small products made of such resins, it is known in the art to employ turret-type molding presses wherein a turntable moves lower mold sections into operative relationship with a fixed lower platen and a vertically movable upper platen. Various vertical and horizontal press arrangements are also known wherein cylinders effect mold closing, and the same or other cylinders apply the requisite clamping pressure. In applications involving larger products, tie rod-type elements or other refinements have been employed to meet clamping force requirements while remaining within reasonable press size specifications. Multi-opening presses, including what are termed double-opening presses, have been employed in the form of side by side openings for products in the nature of sheet materials having very limited thickness. It is also known to provide what are termed stack molds in horizontal thermoplastic injection molding presses which may be operated by rack and pinion drive mechanisms. Due to size, weight, and cost considerations, these known press configurations have not been applicable to designs employing multiple molds for large products or parts of the type herein contemplated. Accordingly, presses for such large products have typically been single mold presses which are limited as noted above in production capacity.